In the case of steps for manufacturing a semiconductor device, positional misalignment between patterns formed by different steps has significant influence on the operation characteristic of the device. Accordingly, techniques for measuring positional relationship of this kind with high accuracy are important to the steps for manufacturing a semiconductor device.
According to an alignment measuring technique for semiconductor devices, a mark belonging to a lower layer and a mark belonging to an upper layer are formed adjacent to each other, and the relative positional relationship between these marks is measured. In general, an optical microscope is used to observe the mark on the lower layer through a film of the upper layer. Accordingly, the film of the upper layer needs to be transparent to light having a wavelength of 300 nm to 1,000 nm (electromagnetic waves), which can be observed by the optical microscope.
However, films used in semiconductor device manufacturing processes have various properties, and thus light permeability cannot necessarily be ensured at the film of an upper layer. For example, wiring layers formed in semiconductor devices are made of a metal, such as copper (Cu) or tungsten (W), which is good in electrical conductivity. Most of the metals used in semiconductor device manufacturing processes are opaque to light.